PTC (positive temperature coefficient of resistance) material has such a property that the electrical resistance thereof steeply increases at a certain temperature. Thus, PTC material is employed for, for example, suppressing short circuit current of a lithium ion secondary battery or serving as a current limiter that can prevent overload current of a motor. Furthermore, PTC material is also used as a heater material that can spontaneously maintain the temperature through passage of current.
As disclosed in Patent Document 1, one well-known PTC material is a barium titanate ceramic material which undergoes change in electric properties at a specific temperature. However, such a barium titanate ceramic material has high electrical resistivity at room temperature. Therefore energy loss involved in current passage is significant. Also, in order to make such a barium titanate ceramic material fit for some uses, lead must be added thereto. That causes a problem against environmental circumstances. In addition, the production cost of the barium titanate ceramic material is high. Thus, alternative PTC materials have been sought.
Under such circumstances, researchers previously found that a composite material formed of a polymer matrix and a conductive substance as an additive exhibits a PTC characteristic. The term “PTC characteristic” refers to such a specific characteristic that the electrical resistivity of the material steeply increases at a specific temperature. Patent Document 2 discloses a composite material which is a mixture of a crystalline polymer (e.g., electrically insulating polyethylene) and conductive particles (e.g., carbon particles). When the mixing ratio is adjusted to a specific value, a conduction path is formed in the polymer matrix of the composite material. That is, at a certain mixing ratio, electrical resistivity drastically decreases as the amount of conductive particles increases.
In the composite material produced so as to have such a mixing ratio, thermal expansion of the polymer matrix is considerably significant as compared with that of the conductive particles. Thus, when the composite material is heated, the crystalline polymer suddenly expands when it melts. The expanding crystalline polymer separates the conductive particles which form a conduction path in the polymer matrix. As a result, the conduction path is cut, to thereby steeply elevate electrical resistivity. Thus, a PTC characteristic is attained.
Meanwhile, a composite material containing an organic material matrix (e.g., a polymer) has poor heat resistance. The material cannot be used in a stable manner in a heater maintained at a high temperature of 150° C. or higher. In addition, since the composite material contains conductive carbon particles, the specific resistivity can be elevated merely to about 1 Ω·cm. Thus, possible use of the composite material is strictly limited.
In order to overcome the aforementioned drawback, there has been developed a composite material formed of a mixture of cristobalite or tridymite with conductive particles. Both cristobalite and tridymite are inorganic materials having high thermal expansion coefficient. Patent Documents 3 to 5 disclose inorganic composite PTC thermistor members, each of which exhibits a room-temperature resistivity about 1/10 to about 1/100 that of a composite material employing, for example, a polymer matrix. Such inorganic composite PTC thermistor members have higher heat resistance, as compared with a PTC thermistor member employing a polymer matrix.